So says Nielsen, tabulating the numbers for the wedding (not the pre- and post-coverage). The official word:
On Friday, April 29, 2011, the much anticipated Royal Wedding of Prince William and Kate Middleton took place at Westminster Abbey. The wedding coverage was carried live from approximately 6AM- 7:15AM ET on 11 networks. In the United States, the sum of the average audience for those networks was 22,766,469 viewers, with a combined household rating of 16.0. The networks carrying live coverage included ABC, CBS, NBC, TEL, UNI, BBCA, CNN, E!, FOXNC, MSNBC and TLC.
While Nielsen did not have an estimated number of viewers for Charles & Diana's nuptials, it had a higher combined household rating, 18.3, during five hours of coverage on three measured TV networks. That was fewer households -- 14.2 million vs. 18.5 million for Will/Kate -- but the population is larger now.
Some comparisons Nielsen offered: The Obama inauguration had a 25.5 rating and 28.9 million viewers across 17 networks for full-day coverage; the latest Super Bowl (a prime-time weekend event) had a 46. rating 53.3 million viewers on one network, and Princess Diana's funeral, an early-morning telecast in the U.S., had a 26.6 rating and 26.1 milliion viewers on eight networks.